1. Field of the Invention
The present invention generally relates to an impedance matching circuit with automatic adjustment and a method thereof and, more particularly, to an impedance matching circuit with automatic adjustment by using a composite resistor unit so as to generate a resistance value within a designed range and an impedance matching method.
2. Description of the Prior Art
For a high-speed transmission interface, as shown in FIG. 1, it is critical for the parameters such as the output impedance (Zout) of the transceiver 11, the input impedance (Zin) of the receiver 12 and the characteristic impedance of the cable 13 to meet one of the international standards so as to prevent the undesired reflection and thus achieve better transmission efficiency during signal transmission. Universal serial bus (USB 2.0) standard, for example, requires a transmission rate of 480 MHz and an output impedance of 45±10% Ω. However, the environment for operating the high-speed transmission interface may vary and the parameters of the interface may not be consistent due to the fabrication process, the output impedance may not be stable and the transmission performance may be adversely affected.
In general, among the above parameters, the output impedance is the most difficult one to keep stable. Therefore, an external device is usually added so as to perform fine tuning on the impedance value. Even though such as method can solve the problem related to the fabrication process; however, it can not overcome the inaccuracy due to the varying environment (such as temperature) during operation.